The invention relates to a cooled continuous casting mould comprising an arrangement for producing a rotating electromagnetic field of force in the unsolidified region of the melt.
It has long been known that it is possible to influence solidification during continuous casting of high-melting metals, by applying rotating electromagnetic fields of force, thus achieving metallurgical and technological advantages, in particular a more uniform grain structure of the cast product. Suitably, the application of the rotating field is effected in the region of the casting level.
The mould material best suited for a rapid formation of a load-bearing skin is copper, to which, for the achievement of improved strength, alloy elements, such as chromium, silver, beryllium, zircon and the like can be added in small amounts. These mould materials, however, strongly shield the force of an applied elecromagnetic field, so that the movement of the liquid melt by the influence of the field does not appear to the desired extent, particularly if the field of force is operated at power line frequency. It is therefore not practical to apply an induction yoke in the region of the mould in which the wall delimiting the inner cavity of the mould is comprised of copper. If, however, a material is used which has a lower electric conductivity than copper and thus a higher permeability to magnetic fields of force, such as brass, aluminium, molybdenum, chromium-nickel-steel and the like, only a short useful life of the mould can be reached, which is not sufficient for production plants.
For avoiding these difficulties it has already been proposed to operate the induction arrangement with a lower frequency, e.g. 1 to 10 Hz, instead of operating it at power line frequency, i.e. 50 to 60 Hz, whereby the losses during the penetration of the copper wall by the field of force will be reduced. Because of this, however, the arrangement will become more complex, since additional frequency transformers are required.
For avoiding the described difficulties it has also been proposed to provide the induction arrangement, not in the region of the continuous mould, but at a smaller or larger distance below the mould. With this arrangement the advantages of influencing the solidification near the casting level and thus producing an advantageous formation of the skin structure, are loss. In particular a more rapid initial solidification, a more finely grained rim structure, a reduced trans-crystalline zone, a lesser sensitivity to tensional cracks, and thus avoidance of surface cracks and internal cracks near the surface, have to be given up. Also a segregation streak will form at the solidification border in the region of the rotating field, which streak reduces the quality of the cast product.
These disadvantages appear all the more, because mould lengths in a range of 600 to 850 mm are typically used for reasons of operational safely, especially with high casting speeds, although a mould length of only 100 to 200 mm is sufficient for the formation of a skin.